$ \left(\dfrac{81}{16}\right)^{\frac{1}{4}}$
Explanation: Figure out what goes in the blank: $\Big(? \Big)^{4}=\dfrac{81}{16}$ Figure out what goes in the blank: $\Big({\dfrac{3}{2}}\Big)^{4}=\dfrac{81}{16}$ So $\left(\dfrac{81}{16}\right)^{\frac{1}{4}}=\dfrac{3}{2}$